1. Field of the Invention
The present invention relates to semiconductor devices utilizing nitride-based compound semiconductor and a method for manufacturing the same.
2. Description of the Related Art
A wide variety of nitride-based compound semiconductor layers are currently available. Semiconductor layers having AlGaN/GaN hetero-structure and layers having AlGaN/AlN/GaN hetero-structure are examples of those layers. To form an ohmic electrode that comes in ohmic contact with those heterostructure semiconductor layer, titanium and aluminum may be used as the material of the electrode. This is described in e.g. Unexamined Japanese Patent Application KOKAI Publication No. H09-008407. Titanium and aluminum have a work function similar to those of the nitride-based compounds.
Manufacturing processes of these semiconductor devices of this kind are, for example, as follows: Initially a semiconductor layer composed of nitride-based compound is formed. Then on the semiconductor layer, an insulation film is deposited. This insulation film is composed of, e.g. SiO2 and/or SiN. Next, of this insulation film, a part on which to build a electrode is removed by photo lithography or other means, thereby exposing the part of the surface of the semiconductor layer. This is in other words forming an opening on the insulation film. Then, a metal film is formed both on the insulation film and on the above-described exposed part of the semiconductor layer exposed via the opening. Subsequently, a heat processing is performed to react the silicon contained in the semiconductor layer with a metal contained in the metal film. This forms a metal alloy electrode.
Most heat treatments for alloying a semiconductor layer, mainly composed of silicon, with a metal film are performed at a temperature of 550 degree. C. or the lower. This heat treatment is performed at a temperature in the range from 440 degree. C. to 550 degree. C. for, for example, 10 to 30 minutes. On the other hand, a higher temperature is required for alloying a semiconductor layer mainly composed of nitride-based compound with a metal film, as compared to the heat treatment for alloying the semiconductor layer composed mainly of silicon with a metal film. Such a heat treatment for alloying the semiconductor layer mainly composed of nitride-based compound requires, specifically, a temperature equal to or higher than 550 degree. C.: for example, a temperature in the range from 550 degree. C. to 850 degree. C.
The metal film is formed also on the insulation film formed on the semiconductor layer. That is, in a certain part the metal film is in contact with the insulation film. This causes a reaction between the metal contained in the metal film and silicon, etc. contained in the insulation film in the heat treatment. The reaction generates a reaction product e.g. AlSi, AlSiO, and Al2O3, etc. This reaction product causes increase of the resistance of the electrode, and increase of the current collapse. In other words, this reaction product deteriorates the electric performance characteristics of the semiconductor device.
When titanium and aluminum are adopted as principal materials of the metal film, the metal film may have a two-layered structure, comprising a lower layer mainly composed of a titanium and an upper layer mainly composed of aluminum. If so configured, the aluminum, which tends to react with the insulation film, can be disjoined from the insulation film. Therefore, to some extent, the generation of the reaction products, which leads to the deterioration of the electric performance characteristics of the semiconductor device, can be suppressed.
However, titanium and aluminum tend to diffuse to each other, and be mixed with each other. Therefore, even the mentioned layer-structured metal film formed of both metals cannot completely suppress the reaction between aluminum, contained in the metal film, and the silicon, etc., contained in the insulation film.
Accordingly, there is a demand for a semiconductor device in which the reaction between a metal, contained in the metal film and silicon, etc., contained in an insulation film, is suppressed, and the method for manufacturing the same. In the so structured semiconductor device, the increase of the resistance of an electrode and the increase of current collapse are suppressed; therefore, the semiconductor device has satisfactory electric performance characteristics.